It is well known to produce man-made vitreous fibers, often described as mineral fibers, by providing a charge of mineral material, melting the charge in a furnace and fiberising the resulting melt to form fibers. The fibers can be used for a variety of purposes, including heat and sound insulation, fire protection, growth substrates, brake linings and vibration control.
The final composition of the fibers is generally expressed in oxides of elements contained in the fibers and it is well established that the composition of the charge of the mineral material, and hence the composition of the melt and the final fibers, can influence use properties of the final fibers.
When formulating a composition for the production of man-made vitreous fibers, it is important to consider not only the properties of the final fibers, but also the melting process, the properties of the melt, and the impact of those properties on the fiberisation process.
The invention relates to man-made vitreous fibers of the stone wool type.
Conventionally, stone wool fibers are fiberized using an external centrifugal process, for example, by use of a cascade spinner. In U.S. Pat. No. 3,159,475 Chen et al. describe such process in general. GB 1,559,117 represents a more extended description. In this type of process, a mineral melt is supplied to the surface of a set of fiberising rotors, which operates in combination with a cold stripping air for fibre drawing to throw off the mineral melt in the form of fibers. The fibers are then carried in an air-flow and collected. A binder is usually applied to the formed fibers and this contributes to the coherence of a finally formed web, which is often generated by consolidation, compression and curing. In some cases, however, no binder is used and the mineral fibers are collected as loose mineral wool.
An alternative fiberisation method is the spinning cup process, which is often referred to as internal centrifugation. In this process, a melt is fiberised by pressing the melt through holes in a spinning cup wall by rotation at high speed to form primary filaments, which then are attenuated to the final fibers by use of a 1300-1500° C. hot air from a burner with excess of oxygen from the combustion. The fibers are subsequently carried in a major air-flow and then collected on a conveyor belt and carried away for further processing to form a man-made vitreous fibre product. The spinning cup process tends to produce man-made vitreous fibre products containing a very low level of unfiberized material, as compared with external centrifugation methods. An additional advantage is that, when collected as a web, the fibers tend to be oriented in the plane of the collector to a greater extent than with external centrifugation methods, which improves the thermal insulation properties of the product. The level of thermal insulation provided is often expressed as a lambda value (λ) (units mW/m·K), which is a measure of the thermal conductivity of the insulation material.
Traditionally, however, internal centrifugal fiberisation processes have only widely been used for fiberising glass wool, which is relatively rich in alkali metal oxides (especially sodium oxide), has a high silica content, low alumina content and includes boron oxide. This traditional glass wool melt has, at reasonably low temperatures (950-1100° C.), all the properties required for the spinning cup method. Traditional stone wool melts, on the other hand, have low silica content, high alumina content and less rich alkali content. These stone wool melt compositions have a significantly higher liquidus temperature than glass wool melts.
For fiberisation in a spinning cup, it is important that the temperature of the melt arriving at the perforated belt of the spinning cup is above the liquidus temperature of the melt composition. This is to avoid crystallisation in the cup during processing. Therefore, in order to process a normal stone wool melt in a spinning cup, it is necessary to fiberize the melt at a higher temperature than glass wool melts. The properties of many stone wool melts at such temperatures are often unsuitable for fiberisation in a spinning cup.
The temperature for the fiberisation process in a spinning cup is often limited between 1150-1220° C., this from both a cost and a construction material point of view. The melt properties of many stone wool melts are often unsuitable for fiberisation in a spinning cup at such temperatures.
Standard stone wool melts can, depending on the melting method, contain significant impurities of metallic iron (Fe(0)). Metallic iron can block the holes in the spinning cup and can also cause corrosion of the spinning cup, increasing the frequency with which the cup needs to be serviced or replaced.
In addition to the properties of a melt like viscosity and liquidus temperature, the properties of the resulting fibers also need to be considered. Of these properties, bio-solubility and high temperature stability are of particular importance.
In recent years, bio-solubility has been added to the criteria that man-made vitreous fibers must meet. That is, the fibers must be able to dissolve rapidly in a physiological medium. For stone wool fibers, the biosolubility relates to the physiological environment in the macrophages in the lungs. It is, therefore, important that there is rapid dissolution at pH 4.5, with the aim of preventing any potential adverse effects from the inhalation of fine fibers.
High temperature stability is also a highly desirable property in stone wool fibers. This is not only in the context of man-made vitreous fibers used specifically in fire protection products, but also in the context of fibers used for thermal or acoustic insulation in buildings.
WO95/01941 describes cupola furnace melts intended for being fiberised in a spinning cup. Whilst the melt has a suitable viscosity and liquidus temperature for use in an internal fiberisation process, the fibers produced have a low level of bio-solubility at pH 4.5 due to the high level of silica in the melt. Furthermore, the cupola melt often contains a measurable amount of metallic iron that leads to a considerable risk of metallic iron droplets clogging up the holes of the spinning cup and thereby stopping the fiberisation process.
In EP1032542, bio-soluble and high temperature resistant fibre compositions are described. A large range for SiO2 and Al2O3 is stated and the compositions must meet the requirements of R2O being at least 10 wt % and 0 wt %<MgO<15 wt %. Many of the examples have a silica content above 43 wt %, and thus only a limited portion of the examples disclosed can be assumed to be bio-soluble at pH 4.5 according to the latest authority requirements. A level of silica of over 43 wt % can be particularly disadvantageous when a high level of MgO is present. A low level of MgO as in the majority of the examples in EP1032542 can result in lower fire resistance. No melting process is specifically described in the document, and the effect of the melting process on the properties of the melt and of the fibers is not recognised.
In EP1667939, bio-soluble, high temperature resistant fibre compositions are described. At least 10% R2O (Na2O+K2O) is required in the composition, which results in high raw material cost, possible emission problems in relation to the melting process and limitations for the high temperature properties of the fibers.
Therefore, whilst previous attempts have been made to provide man-made vitreous fibers that are stable to high temperatures, bio-soluble, and can be produced by a spinning cup method, providing these features in combination whilst keeping the cost of production to a minimum has proved challenging. It would be desirable to provide further man-made vitreous fibre compositions that also meet the above criteria, or even provide an improvement, especially in terms of high temperature stability in combination with biosolubility. It would also be desirable to provide such man-made vitreous fibers in an economical manner, more flexible and efficient production processes and whilst minimising environmental problems associated with emissions.
An object of the present invention, therefore, is to provide a melt composition for the production of mineral fibers having good fire resistance. A further object is to provide a melt composition for the production of mineral fibers having good biosolubility. A further object of the invention is to provide a melt that is suitable for production by known melting technology for stone wool and that is suitable for use in a spinning cup fiberisation method. It is also an object of the invention to provide the melt at low cost. A further object is to minimise problems with emissions. Still a further object of the invention is to provide a process for producing the mineral fibers by the spinning cup method.
A further object of the invention is to provide mineral fibers that are bio-soluble, stable to high temperatures, economical to produce and that contain a low level of unfiberised material.